The present invention relates to a stainless steel plate for a shadow mask on which an etching process can be performed excellently and warp does not occur easily and to a method for producing the same.
The main components constituting a color cathode ray tube of a television receiver include an electron gun, a screen for imaging an electron beam, and a shadow mask as an electrode for selecting colors. In general, the shadow mask uses a thin metal plate of a thickness of 0.3 mm or less on which numerous micro-holes are provided regularly and precisely.
Hitherto, as a material of the metal thin plate for a shadow mask, a low carbon aluminum killed steel (hereinafter low carbon steel will be referred to) has been used.
However, in this material, a long time of irradiation with electron beams due to a continuous use causes a thermal expansion, thus distorting the micro holes provided on the plate. As a result, the misalignment of colors, called a doming phenomenon, occurs so that electron beams passing through the micro-holes are mislocated from the predetermined phosphor dots.
In particular, recently, since a large size and high quality of color television, or high accuracy of personal computer displays are demanded, the above-mentioned doming phenomenon becomes a large problem.
Therefore, for such applications of use, Fexe2x80x94Ni invar alloy (hereinafter, invar alloy will be referred to), which has a small thermal expansion of about {fraction (1/10)} of a common steel, has been used widely.
However, since the invar alloy is an expensive metal material, it is not appropriate from the economical viewpoint.
On the other hand, recently, a flat television in which an image appears on a display screen apparently and is recognized visually has been given attention.
In this method, since a shadow mask is incorporated into a cathode ray tube so that a shadow mask is held with tension applied, the deformation of the shadow mask due to the thermal expansion can be prevented. Therefore, in the material having a coefficient of thermal expansion larger than that of the conventional invar alloy, the doming phenomenon does not easily occur.
However, on the contrary, since high tension is applied to the shadow mask itself, a metal material with high strength is required.
When the shadow mask is incorporated into the cathode ray tube, the shadow mask is subjected to a heating process of about 500xc2x0 C. with tension applied. Therefore, the shadow mask is required to be produced of a material that is not deformed easily at high temperature.
Furthermore, since a low carbon steel or invar alloy, which has been used conventionally, is poor in corrosion resistance and easily rusts, such materials have to be stored generally in a state where they are coated with a rust-preventive agent. Therefore, a material for a shadow mask that does not form rust easily and has an excellent corrosion resistance even during storage has been highly demanded.
Moreover, JP63-255340A proposes an Fe-based material including 1.0 to 4.0% of Cu (hereinafter, component rate is expressed by %, and % means weight % unless otherwise noted) as a material for a flat tension shadow mask having a high proof strength so that deformation does not occur easily at the time of fabrication or in use and sufficient elastic stretchability so that plastic deformation does not occur due to the thermal distortion in use.
However, although this metal material has 0.2% proof strength of 50 kgf/mm2 (490 MPa) or more, the coefficient of thermal expansion is substantially the same as that of low carbon steel. Therefore, this material cannot prevent the doming phenomenon sufficiently.
Furthermore, this material has a corrosion resistance substantially the same as that of low carbon steel or invar alloy, and likewise requires coating with rust-preventive agent during the storage.
However, in order to provide micro-holes on a metal thin plate for a shadow mask, it is common to employ a photo-etching process utilizing a corrosion melting phenomenon of the metal. The photo-etching process is carried out by:
(a) degreasing and washing a metal thin plate to form a photosensitive photoresist film on the surface of the metal thin plate and thermosetting a predetermined pattern;
(b) then developing this pattern into the intended form of photoresist patterns;
(c) spraying a solution of ferric chloride on the surface of the metal thin plate on which the photoresist patterns are developed and melting an exposed metal part so as to provide micro-holes; and
(d) finally removing the photoresist film.
Thus, the intended shadow mask can be obtained. However, in the process in which the metal thin plate is subjected to corrosion melting by the etching process, as shown in a cross section in FIG. 1, corrosion in the side direction, called side etching (S), simultaneously proceeds in addition to the corrosion in the depth (D) direction. In FIG. 1, reference numeral 1 denotes a metal thin plate, 2 denotes a photoresist film, and 3 denotes an etched hole.
Herein, a value obtained by dividing the etching depth (D) by side etching (S) is called an etching factor.
Namely, in the schematic view of the etched cross section of FIG. 1, the etching factor (EF) is represented by the following formula 1:
EF=D/S=D/[(Wxe2x88x92M)/2]xe2x80x83xe2x80x83(formula 1)
wherein
M: width of pattern of the photoresist film
W: width of groove after etching process
S: side etching
D: depth after etching process.
In order to provide micro-holes as on a shadow mask by the photo-etching process, the above-mentioned side etching should be as little as possible. Therefore, it is desirable that a metal material has a large etching factor (EF).
Furthermore, if there is a large amount of inclusions in the steel, when the etching process is performed, the neighborhood of the inclusion is nonuniformly dissolved, thus making the porous shape irregular. Therefore, when such a metal is used, it is difficult to provide micro-holes as on a shadow mask. Therefore, it is a necessary condition as a material for a shadow mask that a material includes as few inclusions as possible.
A metal thin plate that is a material for a shadow mask is generally produced by forming a material metal into a plate material, and cold rolling and annealing of the plate material. Since the annealing state may be insufficient in mechanical strength, it is common to perform temper rolling.
Furthermore, the flatness is poor in the metal plate that is subjected to the temper rolling, uniform tension cannot be applied to the plate and the plate is wrinkled. In such a case, occasionally, in order to correct the plate shape, bending and restoring are done repeatedly so as to carry out the shape correction (tension level controller) of the plate.
However, in the metal plate that is subjected to the cold rolling or shape correction as mentioned above, although the plate appears flat, warp occurs as the removal of the plate thickness from one side of the plate by etching process (half etching) proceeds.
In particular, in the metal plate after its shape are corrected, although the flatness is improved as compared with the plate which no processing is performed after cold rolling, the warp may be larger at the time of etching process.
Namely, micro-holes provided on the shadow mask is designed to have a small apertured portion (small hole 4) at the side of the electron gun (the side where electron beams enter), and a large apertured portion (small hole 5) at the side of the phosphor screen (the side where electron beams are emitted), so that the electron beams are introduced into the predetermined phosphor screen exactly. However, when micro-holes are provided on the metal plate for a shadow mask produced by a cold rolling such as a temper rolling or a shape correction in accordance with a usual method of etching process, warp tends to occur disadvantageously.
If the shadow mask warps, the disadvantage in working occurs, for example, bending in handling etc. easily occurs during handling, or it is necessary to include a step for modifying the warp shape when masks are set.
As the effective means for preventing warp generated when asymmetric etching processes are performed with respect to a rear side and a front side of the metal plate, for example, annealing treatment of a metal plate after the cold rolling is performed at the temperature below the softening temperature with tension of yield point or less applied (so-called, a tension anneal method) is well known. According to this method, it is possible to correct the flatness of the steel belt and at the same time to reduce the residual stress.
However, the tension anneal method requires an apparatus for applying tension or equipment resistant to the high tensile strength. Therefore, expensive and specifically designed equipment is required.
Therefore, a method for producing a metal plate for a shadow mask in which warp dose not occur after the etching process has been demanded.
On the other hand, a material of the metal plate for a shadow mask has been mainly a low carbon steel plate (low carbon aluminum killed steel), however, in this material, a long time of irradiation with electron beams due to a continuous use causes a thermal expansion, to thus distort the micro holes provided on the plate. As a result, the misalignment of colors called a doming phenomenon occurs so that electron beams passing through the micro-holes are mislocated from the predetermined phosphor dots.
Therefore, invar alloy (Fexe2x80x94Ni invar alloy) having a small thermal expansion (about {fraction (1/10)} as that of common steel) has been used as a shadow mask. However, this invar alloy is expensive, so that it is not appropriate from the economical viewpoint.
Furthermore, a flat television in which an image appears on a display screen apparently and recognized visually has been given attention. In this method, since a shadow mask is incorporated into a cathode ray tube so that a shadow mask is held with tension applied, the deformation due to thermal expansion can be prevented. Further, even in the material having a larger thermal expansion than the conventional invar alloy, it is advantageous that the doming phenomenon hardly occurs. However, on the contrary, since high tension force is applied to the shadow mask itself, a metal material with high strength is required. Furthermore, when the shadow mask is incorporated into the cathode ray tube, the shadow mask is subjected to a heating process of about 500xc2x0 C. with tension applied. Therefore, the shadow mask is required to be formed of a material that is not plastic deformed easily at high temperature.
Furthermore, since the conventionally used low carbon steel or invar alloy is sufficient in corrosion property and easy to form rust, such materials have to be generally stored in state where they are coated with a rust- preventive agent. Therefore, a material for a shadow mask, which does not easily form rust when it is stored and is excellent in corrosion resistance, has been high demanded.
Moreover, JP63-255340A suggests a metal material including 1.0 to 4.0% of Cu and the rest including Fe and inevitable impurities as a material for a flat tension shadow mask having a high proof strength in which deformation does not occur during the fabrication or in use and sufficient elastic stretchability in which plastic deformation does not occur due to the thermal distortion in use. Although this material is characterized in that the 0.2% proof strength is 50 kgf/mm2 (490 MPa) or more, the coefficient of thermal expansion is substantially the same as that of low carbon steel plate. Therefore, the doming phenomenon cannot suppress sufficiently. Furthermore, the corrosion property is the same as that of low carbon steel or invar alloy. Also this material needs to be coated with the rust-preventive oil during the storage.
Of course, in a metal plate for a shadow mask on which micro-holes are provided by the photo-etching method, the above-mentioned side etching should be as small as possible. Accordingly, as mentioned above, a metal material having a large etching factor (EF) is desirable.
Furthermore, if there is a large amount of inclusions in metal material, when the etching process is performed, the parts around the inclusions are nonuniformly melted, and thus the hole shapes are irregular. Thus, it is difficult to perform micro-holes. Therefore, a material including extremely few inclusions is also desirable for the material for a shadow mask.
Under such circumstances, there has been increasing demand for a metal plate for a shadow mask in which the coefficient of thermal expansion is smaller than that of the low carbon steel plate material, the price is less expensive than the invar alloy material, the amount of plastic deformation is small at high temperature, mechanical strength is so excellent that the plate can be used for a shadow mask used with high tension applied and furthermore, etching property is excellent.
It is therefore an object of the present invention to provide a stainless steel plate for a shadow mask in which the coefficient of thermal expansion is smaller than that of the low carbon steel plate material, the price is less expensive than the invar alloy, the amount of plastic deformation is small at high temperature, and mechanical strength is sufficiently high so that the plate can be used for a shadow mask used with high tension applied, further the etching property is excellent, and the shape is stable after the etching process; a method for producing the same; and a shadow mask.
The present inventors intensively investigated and found that when the specified amount of Cr, a small amount of C, a small amount of Mn, Ti, Si or Al if necessary, are contained in Fe, and the contents of inevitable impurities P and S are set to be low, it is possible to obtain a stainless steel suitable for a shadow mask material, in which the coefficient of thermal expansion is small, and mechanical strength, etching property, and corrosion resistance property are excellent.
Furthermore, it is possible to obtain the following findings (a) to (c).
(a) In a metal plate that is subjected to cold rolling such as temper rolling, since the surface predominantly is stretched by a roller, stretching stress is accumulated on the surface layer of the metal plate as an internal stress. Furthermore, also in a metal plate that is subjected to the shape correction by a tension level controller, etc., a compressive stress is accumulated on the surface of the metal plate due to the bending and restoring process. As mentioned above, in the metal plate on which the internal stress is accumulated, although the plate appears flat, warp occurs as the plate thickness is removed from one side of the plate by etching (half etching). Because of the loss, the stress corresponding to the melted and released plate thickness, thus the stress of the front surface and rear surface get out of balance, which may be lead to warp on the plate. In particular, as explained in FIG. 2, micro-holes provided on the metal plate for a shadow mask is designed to have a small apertured portion on one side, and a large apertured portion on the other side. Therefore, in order to provide such micro-holes, a metal plate for a shadow mask that is subjected to cold rolling such as temper rolling or shape correction is etched, and the amount of the accumulated stress due to melting becomes asymmetric between the large apertured portion and the small apertured portion. Consequently, the stress gets out of balance, thus to generate a remarkable warp.
(b) However, even in the metal plate for a shadow mask that is subjected to cold rolling such as temper rolling or shape correction and in which the internal residual stress is accumulated, when the metal plate is annealed at a low temperature before a recrystallization, the internal residual stress is sufficiently relaxed. Therefore, warp does not occur even if the micro-holes asymmetric between the front side and the rear side are provided by an etching process. Furthermore, the mechanical strength necessary to the shadow mask is not affected.
(c) Furthermore, as a metal plate for a shadow mask, a stainless steel containing the specified amount of Cr, a small amount of C, and a small amount of Mn, Ti, Si or Al, if necessary, and setting the contents of inevitable impurities P and S to be low is employed, it is possible to obtain a material for a shadow mask having a small coefficient of thermal expansion and excellent mechanical strength, etching property (fine etching process, uniformity of hole shape) and corrosion resistance.
The present invention is completed based on the above-mentioned findings. According to the present invention, a stainless steel plate for a shadow mask includes 9 to 20 weight % of chromium (Cr), 0.15 weight % or less of carbon (C), 0 to 1.0 weight % of manganese (Mn), 0 to 0.2 weight % of titanium (Ti), 0 to 1.0 weight % of silica (Si), and 0 to 1.0 weight % of aluminum (Al); wherein the rest includes ferrite (Fe) and inevitable impurities, and in the inevitable impurities, the content of phosphor (P) is 0.05 weight % or less and the content of sulfur (S) is 0.03 weight % or less.
Furthermore, according to the present invention, a method for producing a stainless steel plate for a shadow mask including 9 to 20 weight % of chromium (Cr), 0.15 weight % or less of carbon (C), 0 to 1.0 weight % of manganese (Mn), 0 to 0.2 weight % of titanium (Ti), 0 to 1.0 weight % of silica (Si), and 0 to 1.0 weight % of aluminum (Al); wherein the rest includes ferrite (Fe) and inevitable impurities, and in the inevitable impurities, the content of phosphor (P) is 0.05 weight % or less and the content of sulfur (S) is 0.03 weight % or less, includes annealing the metal plate for a shadow mask after cold rolling or shape correction is performed at the end-point temperature of the plate of 550 to 650xc2x0 C.